71 Aromaticity
Aromaticity in organic chemistry does not refer to whether or not a molecule triggers
a sensory response from olfactory organs (whether it ”smells”), but rather refers to the
arrangement of electron bonds in a cyclic molecule. Many molecules that have a strong
odor (such as diatomic chlorine Cl 2 ) are not aromatic in structure -- odor has little to
do with chemical aromaticity. It was the case, however, that many of the earliest-known
examples of aromatic compounds had distinctively pleasant smells. This property led to
the term ”aromatic” for this class of compounds, and hence the property of having enhanced
stability due to delocalized electrons came to be called ”aromaticity”.
71.1 Definition.
Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone
pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the
stabilization of conjugation alone. It can also be considered a manifestation of cyclic delo-
calization and of resonance.
This is usually considered to be because electrons are free to cycle around circular arrange-
ments of atoms, which are alternately single- and double-bonded to one another. These
bonds may be seen as a hybrid of a single bond and a double bond, each bond in the ring
identical to every other. This commonly-seen model of aromatic rings was developed by
Kekulé. The model for benzene consists of two resonance forms, which corresponds to the
double and single bonds’ switching positions. Benzene is a more stable molecule than would
be expected of cyclohexatriene, which is a theoretical molecule.
71.2 Theory
By convention, the double-headed arrow indicates that two structures are simply hypothet-
ical, since neither can be said to be an accurate representation of the actual compound. The
actual molecule is best represented by a hybrid (average) of most likely structures, called
resonance forms. A carbon-carbon double bond is shorter in length than a carbon-carbon
single bond, but aromatic compounds are perfectly geometrical (that is, not lop-sided) be-
cause all the carbon-carbon bonds have the same length. The actual distance between
atoms inside an aromatic molecule is intermediate between that of a single and that of a
double bond.
AbetterrepresentationthanLewisdrawingsofdoubleandsinglebondsisthatofthecircular
πbond(Armstrong’s innercycle), in whichtheelectrondensityisevenlydistributed through
aπ bond above and below the ring. This model more correctly represents the location